Temperature detectors



July 24, 1962 R. GRIFFITHS 3,045,326

TEMPERATURE DETECTORS Original Filed April 28, 1955 INv ENT R ATTORNEYUnited States Patent 3,045,326 TEMPERATURE DETECTORS Rupert Grifliths,London, England, assignor of one-half to Graviner Manufacturing CompanyLimited and onehalf to Wilkinson Sword Limited, both of London, England,and both British companies Original applicationApr. 28, 1955, Ser. No.504,630, now Patent No. 2,836,692, dated May 27, 1958. Divided and thisapplication Mar. 21, 1958, Ser. No. 722,914 Claims priority, applicationGreat Britain May 7, 1954 3 Claims. (Cl. 29'--155.5)

This invention relates to temperature detectors and to methods ofmanufacture thereof.

One particular use of temperature detectors constructed in accordancewith the present invention is in aircraft, where they may be used fordetecting the existence of a predetermined temperature, for example toindicate the presence of flame or of overheat conditions. Such detectorsmay also be used equally well for non-aircraft purposes.

According to the present invention there is provided a temperaturedetector comprising two spaced electrical conductors the space betweenwhich contains a temperature sensitive material whose electricalresistance falls with rising temperature, said temperature sensitivematerial softening at elevated temperatures, said space also containingdiscrete particles of refractory material distributed therein foropposing movement together of said conductors when said temperaturesensitive material is softened.

The invention also provides a temperature detector comprising twoelectrical conductors spaced apart by a temperature sensitive materialwhose electrical resistance varies appreciably with variations intemperature over a predetermined temperature range, said temperaturesensitive material softening at elevated temperatures, the space betweensaid conductors also containing discrete particles of a material whichdoes not soften appreciably at said elevated temperatures, saidparticles serving to oppose movement of said conductors into contactwith one another when said temperature sensitive material is in asoftened condition.

The invention further provides a temperature detector comprising twoelectrical conductors separated by a glasslike substance whoseelectrical resistance falls substantially at temperatures at whichdetection is desired, the space between said conductors also containingdiscrete particles of a material which does not soften appreciably attemperatures at which said glass softens whereby said particles serve tooppose movement of said conductors into contact with one another whensaid glass is softened.

Whilst such a detector may be manufactured by various methods, onesuitable method comprises fusing a tube of glass-like materialprogressively from one end whilst an elongated electrical conductorpassing through the interior of the tube is moved relative to the fusedportion of the tube to cause fused glass to be drawn from the tube toform a coating on the conductor, feeding said discrete particles intothe interior of the tube whereby said particles are embedded in theglass-like coating formed on said conductor, and thereafter applying asecond elongated electrical conductor to the outside of said glass-likecoating.

For such methods of manufacture there may be provided apparatuscomprising means for supporting the tube of glass-like material, meansfor fusing said tube, means for drawing an electrical conductor throughthe interior of said tube, means for moving said tube relative to saidfusion source as fused material is drawn away on said conductor wherebythe supply of fused glass is maintained,

3,045,326 Patented July 24', 1962 and means for supplying said discreteparticles to the space between said tube and said electrical conductor.

A preferred construction of temperature detector in accordance with thepresent invention and methods of manufacture thereof will now bedescribed, by way of example only, with reference to th accompanyingdiagrammatic drawings in which:

FIGURE 1 is a sectional side elevation of a small part of a completeddetector;

FIGURE 2 shows the final stage in one method of manufacture of thedetector;

FIGURE 3 is a side elevation, partly in section, of one form of appartusfor producing the detector, whilst FIGURE 4 is an enlargedcross-sectional view showing one stage of production of the detector.

The present application is a division of my prior application SerialNumber 504,630 filed April 28, 1955 now Patent Number 2,836,692 issuedMay 27, 1958.

In its preferred form as shown in FIGURE 1 the detector comprises aninner wire conductor 10, an outer conductive sheath 1-1, a filling oftemperature sensitive material 12 and discrete particles 13 ofrefractory material embedded in the temperature sensitive material 12.Normally, a plug or socket fitting of any suitable form is provided ateach end of the finished detector.

In this construction the temperature sensitive material 12 is glass,this term being intended to embrace all glasslike substances. Manydifferent varieties of glass may be used in the detector provided thatthey possess the property that their electrical resistance is extremelyhigh at low temperatures so that they are virtually a good insulatingmaterial, but have an electrical resistance which decreases with risingtemperature and eventually falls to a relatively low value so that theglass is virtually a good electrical conductor. Detection is normallyeffected by applying a potential between thetwo conductors and utilisingthe passage of current between the two conductors, occurring at thepredetermined temperature at which the resistance has fallen to a lowvalue, to effect operation of a suitable indicator or warning device.The temperature versus resistance characteristic of such a glass isdependent upon its composition, so that by using an appropriatecomposition detection at a desired temperature may be obtained.

Examples of suitable glasses for this purpose are a' soda glass known asX8, a lead glass known as Ll, both these glasses being available fromMessrs. Plowden and Thompson, and a glass sold by that same firm underthe registered trade-mark Kodial." Examples of materials which may beused for the inner conductor are an %-20% nickel-chromium alloy wireknown as Nichrome or a borated copper-clad nickel-iron Wire of the kindcommonly used in electric lamp production, a stainless steel sheathbeing used for the outer conductor. Other materials which may be usedfor the conductors are wire and tube consisting of nickel-iron alloys,for example those known as Nilo K or Nilo 50.

It is a requirement for aircraft detectors that they should be capableof withstanding a temperature in excess of one thousand degreescentigratde for several minutes, and still function correctlythereafter. At such temperatures the glass will become relatively soft,and there is a danger that the two conductors may move into contact andthe detector will then be useless. It is in order to prevent, or atleast reduce the likelihood of, contact between the two conductors thatthe discrete particles 13 of refracatory material are provided in thespace between the two conductors. Various materials may be used for thispurpose, provided that they are sufficiently refractory not to softenappreciably at the maximum temperature which the detector is designed towithstand. One substance which we have found to be 3 suitable for thispurpose is alumina of the grade sold by The Turret Grinding WheelCompany under the designation No. 120 grit.

In the manufacture of a detector having the form shown in FIGURE 1 it isquite possible by using a suitable fusion source to apply the glasscoating, with the discrete particles therein, to the wire by manualmovement of the wire relative to the fused glass. Thus the innerconductor may be threaded into a length of glass tubing and therefractory particles dispersed in the space between the glass tubing andthe wire. The glass tubing is then heated progressively from one end tosoften the glass whilst the wire is pulled through the glass tube todraw down the glass onto the wire. During the drawing down of the glassonto the wire the refractory particles become embedded in the glass. Asindicated in FIG- URE 1 this method results in the concentration ofrefractory particles being highest in the vicinity of the innerconductor, but such distribution is not an essential. By correctlyrelating the rate of fusion of the glass to the speed of relativemovement between the wire and the glass, a substantially uniform coatingof glass with the particles embedded therein can be formed. If a coaxialdetector is to be produced the coated wire is then threaded into theouter conductive sheath 11 which is subsequently brought into intimatecontact with the glass coating by any suitable reducing process; forexample the sheath may be drawn down by passing it through one or moredies, such as the die 14 shown diagrammatically in FIGURE 2, or may bereduced by swaging or pressing. In order to ensure satisfactory contactbetween the glass and the inner conductor it may be necessary to refusethe coating on the wire after its initial formation, and the detectormay again be heated during or after the application of the outerconductor in order to give better contact between that conductor and theglass coating.

By way of example only, it may be stated that detectors according to theinvention have been made having an outside diameter of between 0.065 and0.09 of an inch, the inner conductor being of No. 29 Standard WireGauge.

It is clearly desirable to mechanise the above described method ofmanufacture of detectors, and whilst this can be done in different waysone suitable apparatus for the purpose is shown in FIGURE 3. Thisapparatus comprises a stand 20 which carries a lead screw 21 on which ismounted a carrier 22 for supporting the glass tube 23. Fixed below theend of the lead screw 21 is a fusion source 24, which may be gas fired.The inner conductor in the form of a wire 25 is drawn from a reel 26which is tensioned by a spring loaded brake 27, the wire being littleabove the fusion source 24. The hopper 29 contains the refractoryparticles 31 which fall through the feed tube 30. To prevent jamming orsticking of the particles a vibrator 32 is connected to the tube 30 tovibrate it at a suitable frequency and low amplitude.

To start the manufacturing process the end of the glass tube 23 is fedinto the fusion source 24, and when the end of the glass is fused theWire 25 is pulled downwards to draw down the fused glass onto the wire.Draw-off rollers 33 are then brought into engagement with the firstportion of coated wire, the draw-off rollers 33 being driven from amotor 34 to continue the drawing down of the coated wire. wire from theend of tube 23 it is necessary to move the tube 23 downwards into thefusion source, and for this purpose the lead screw 21 is driven from themotor 34 via a variable ratio gear box 35 in order to move the carrier22 downwards at an appropriate rate. As shown in FIGURE 4, which is anenlarged view of the tube 23 and wire 25 in the vicinity of the fusionzone, the diameter d of the coating is very much smaller, for ex- Asglass is drawn ofi the ample approximately one sixth, of the diameter Dof the tube 23 so that the required speed of downward movement of theglass tube 23 is very much lower than the required speed of the coatedwire. The ratio between the two speeds can be adjusted by means of thegear box and the fact that both movements are effected from a commondriving source helps to ensure an approximately constant coatingthickness. If desired, an optical projection device 36 may be providedfor giving a magnified image of the coating to allow visual monitoringwhich can be followed by adjustment of the apparatus if necessary.

If it is found necessary to re-fuse the coated wire to ensure wetting ofthe wire by the glass throughout its length, this can be achieved byproviding a further fusion source 37 below the rollers 33. The coatedwire is then directed by a suitable supporting guide 38 onto a bench 39where it may be cut into suitable lengths. The guide should have asufficiently large radius to prevent damage to the coated wire, thecoating of which may not at this stage have solidified throughout. Afterthe coated wire has been cut it may be sheathed by any suitable method,for example by the methods mentioned above. Of course, if desired thecoated wire could be sheathed as it is drawn from the coating apparatusbut no particular advantage can be seen in making this stage of themanufacture continuous with the previous sta'ge.

Whilst the above description and drawings have referred specifically toa detector in which the two conductors are coaxial, it should beunderstood that the invention is not limited to this arrangement of theconductors, but that both conductors may be in the form of wires. Aftercoating these wires with the glass, which has the refractory particlesdistributed therein, the coated wires may subsequently be introducedinto an outer sheath which forms an envelope for the temperaturesensitive material but does not function as a conductor. With thisconstruction it may be desirable to pack further material into thesheath after the coated wires have been introduced, to fill any vacantspace in the sheath.

What I claim is:

l. The method of producing an elongated temperature change detectorhaving an inner electrical conductor surrounded by glass compositionmaterial enclosed within another electrical conductor, temperaturechanges being indicated by associated changes in the electricalconductivity of said glass material, comprising the steps of threadingone end of an elongated electrical conductor having a cylindrical outersurface through a tube of glass composition material having a boregreater than the maximum cross-sectional dimension of the conductor,heating that end of the glass tube adjacent said one end of theelectrical conductor to soften said glass until it contacts saidelectrical conductor, moving said one end of the electrical conductoraway from the softened end of the glass tube whereby softened glass isdrawn away from the tube to form a coating on the electrical conductor,progressively extending the heating of said tube away from the initiallyheated end to maintain a supply of softened glass, feeding finelydivided particles of a refractory material having a softeningtemperature appreciably in excess of the softening temperature of theglass composition material into the region in the interior of the glasstube where said glass is softened, whereby particles of the refractorymaterial are embedded in the glass coating formed on the conductor, saidrefractory material having an electrical conductivity which isnegligible by comparison with that of the glass within the range oftemperature changes to be detected, and applying a second elongatedelectrical conductor to the outside of the glass coating on the firstmen tioned elongated conductor.

2. The method of producing a temperature change detector in the form ofa readily bendable electric cable having an inner electrical conductorsurrounded by glass composition material enclosed within anotherelectrical conductor, temperature changes being indicated bycorresponding changes in the electrical conductivity of said glassmaterial, said glass material having embedded therein finely dividedparticles of a refractory material which has a softening temperatureappreciably in excess of the softening temperature of the glasscomposition material, and which has an electrical conductivity which isnegligible by comparison with that of the glass material at least up tothe softening temperature of the glass material, comprising the steps ofpassing a leading end of an elongated electrical conductor through atube of glass composition material, said conductor having a cylindricalouter surface and said tube having a bore greater than the maximumcross-sectional dimension of the conductor, heating the glass tube atthe end adjacent said leading end to soften the glass until it contactssaid conductor, moving the leading end of said electrical conductor awayfrom the softened end of the glass tube whereby softened glass is drawnfrom the tube to form a coating on the electrical conductor, feedingfinely divided particles of refractory material into the interior of theglass tube concurrently with the step of moving the leading end of saidconductor away from the softened end of the glass tube whereby particlesof the refractory material are embedded in the glass coating formed onthe conductor, continuing the heating of the glass tube progressivelyalong its length as fused glass is withdrawn on the conductor whilstcontinuing the movement of the leading end of the conductor away fromthe softened end of the tube, and applying a second elongated electricalconductor to the outside of the glass coating on the first mentionedelongated conductor.

3. The method of producing a temperature change detector having anelongated inner electrical conductor surrounded by glass enclosed withinan elongated tubular electrical conductor, temperature changes beingindicated by changes in the electrical conductivity of said glassmaterial, said glass material having embedded therein finely dividedparticles of a refractory material which has a softening temperatureappreciably in excess of the softening temperature of the glasscomposition material, and which has negligible electrical conductivitywithin the range of temperature change to be detected comprising thesteps of heating one end of a tube of glass composition material tosoften said end of the tube, moving an electrical conductor through saidsoftened end to draw fused glass from the tube onto the conductor toform a glass coating on the conductor, said conductor having acylindrical periphery, feeding finely divided particles of refractorymaterial into the interior of the glass tube at the softened end wherebyparticles of the refractory material become embedded in the glasscoating formed on the conductor, continuing the heating of the glassprogressively along its length as fused glass is withdrawn on the movingconductor, re-heating the glass coating on the conductor, passing there-heated glass coated conductor into a tubular elongated electricalconductor, and, finally drawing down the tubular conductor to bring itinto contact with the glass coating on the elongated inner conductor.

References Cited in the file of this patent UNITED STATES PATENTS1,359,400 Lightfoot Nov. 16, 1920 1,987,915 Smith Ian. 15, 19352,263,601 Wendler Nov. 25, 1941 2,344,648 Simmons Mar. 21, 19442,367,211 Greenfield Jan. 16, 1945 2,637,797 Schenk May 5, 19532,677,172 Oakley May 4, 1954 2,789,194 Gosmann Apr. 16, 1957 2,884,689Wondra May 5, 1959 2,893,182 Pies July 7, 1959 FOREIGN PATENTS 12,886/33Australia May 31, 1933 471,252 Great Britain Sept. 1, 1937

